Historically, the overall risk of infant mortality in the United States has declined. However, the rate of decline has slowed over the last 20 years and racial and ethnic disparities have increased. Consequently, infant mortality remains a serious public health problem. Moreover, the cause of the racial and ethnic differentials has remained elusive. The objective of this proposal is to develop the methodological tools necessary to better understand the factors, biological, environmental, economic and social that influence infant mortality and racial and ethnic disparities in infant mortality. It is our view that effective theoretical tools, such as the proximate determinants model of infant and childhood mortality, are available, but that the statistical tools to fully operationalize these models are not available. Ideally, these statistical tools should be able to accommodate direct and indirect (through the proximate determinants) effects, allow for non-linear effects, appropriately parameterize the proximate determinants (the most important of which are birthweight and gestational age), and accommodate unobservable heterogeneity in the birth cohort. Our preliminary work indicates that a combination of mixture models and logistic regression can achieve all of these ends. To date we have developed and tested mixture models of birthweight and gestational age (with and without covariate structures), and modeled the relationship of both birthweight and gestational age with mortality. We will extend these results here to test alternative parameterizations of the mixture model and develop multivariate mixture models that combine birthweight and gestational age with mortality. We plan to conduct a power analysis to determine the optimum sample sizes necessary for operationalizing the proximate determinants model of infant mortality. The result will be a well-documented statistical method that fully operationalizes the proximate determinants model of infant and childhood mortality. This will be invaluable for testing theories concerning the causes of the decelerating decline of infant mortality, as well as, the increases in racial and ethnic differentials in infant mortality. Finally, we will conduct an illustrative analysis using this model to evaluate the effects of maternal age and parity on infant mortality including the direct and the indirect effects of maternal age and parity through the proximate determinants of birthweight and gestational age.